Thermodynamic activity | EPFL Graph Search

In chemical thermodynamics, activity (symbol a) is a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution. The term "activity" in this sense was coined by the American chemist Gilbert N. Lewis in 1907. By convention, activity is treated as a dimensionless quantity, although its value depends on customary choices of standard state for the species. The activity of pure substances in condensed phases (solid or liquids) is normally taken as unity (the number 1). Activity depends on temperature, pressure and composition of the mixture, among other things. For gases, the activity is the effective partial pressure, and is usually referred to as fugacity. The difference between activity and other measures of concentration arises because the interactions between different types of molecules in non

Etude des propriétés morphologiques et catalytiques d'agrégats d'or triés en taille et déposés sur le TiO2(110)

Raphaël Vallotton

This thesis aims to find, for the first time, a direct relation between the size and morphology of small metallic nanostructures (gold in this case) supported on a metal-oxide surface to their catalytic activity. In this perspective, three main topics have been treated during this thesis. The first part concerns the design and realization of a Scanning Tunneling Microscope (STM) supposed to work over a wide temperature range (4K < T ≤ 300K). This new device replaces an existing solution. The coarse approach of the scanning tip towards the sample is realized by an axial motor based on a sapphire prism gliding on shear piezos in the stick&slip mode. The new STM is very rigid moving the resonance frequencies to higher values with respect to the existing solution. Operation of the motor down to T = 8K has been proven, however topographic imaging has been performed only in the temperature range 77K < T ≤ 300K. The second part of this work focuses on a study of the evolution of the morphology of gold nanoparticles on a TiO2(110) surface. Size-selected clusters Aun+ (n = 5, 7) are deposited at a well defined kinetic energy on the surface held at room temperature. Subsequent annealing of the sample has been performed stepwise. After each temperature increase, the morphology has been determined by STM. The evolution as a function of surface temperature has been studied for two different surface reconstructions, TiO2(110)-(1×1) and TiO2(110)-(2×1). The deposition process leads only to small fragmentation and the morphology is stable up to T = 400K. Further increase of the surface temperature leads to sintering of the particles by Ostwald ripening as shown by an exponential decrease of the island density with temperature. The main topic of this thesis, the correlation between morphology and catalytic activity, is described in the last part of this manuscript. For the first time we are able to relate the onset of CO2 production from CO and O2 to a clear change in the morphology. The catalytic activity of the particles strongly depends on their size and dimensionality. The relative activity per particle has been determined and we find a clear maximum for clusters containing 60 atoms and are 3 to 4 monolayers high. These results are discussed in contrast of literature data on the same but also on different metal-oxide surfaces.

EPFL2009

Biomimetic catalysis with immobilized transition metal complexes

Alexander Schiller

The tetradentate chelate ligand tris[(1-vinylimidazol-2-yl)methyl]amine (10) was synthesized in five steps from commercially available starting materials. Upon reaction with MCl2 (M = Zn, Cu, Ni, Co) in the presence of NH4PF6, the complexes [M(10)Cl]PF6 (M = Zn (11), Cu (12), Co (14)) and [Ni(10)Cl]2(PF6)2 (13) were obtained. The structure of all complexes was determined by single crystal X-ray crystallography. Immobilization of 11 and 12 was achieved by copolymerization with ethyleneglycol dimethacrylate (EGDMA) to obtain P11-Zn and P12-Cu. Immobilized versions of 13 and 14 have been generated by cleavage the metal from P12-Cu and subsequent assembly of MCl2 (M = Ni (P12-Ni), Co (P12-Co)). The supported complexes P11-Zn, P12-Cu, P12-Ni, and P12-Co were found to be efficient catalysts for the hydrolysis of bis(ρ-nitrophenyl)phosphate (BNPP) at 50 °C. At pH 9.5, the heterogeneous catalyst P12-Cu was 56 times more active than the homogeneous catalyst 12. Partitioning effects, which increase the local concentration of BNPP in the polymer, are shown to contribute to the enhanced activity of the immobilized catalyst. The tridentate N,N',N''-chelate ligand tris(1-vinylimidazol-2-yl)phosphine (20) was synthesized from commercially available starting materials. Three immobilized Cu(II) complexes were generated by the following: (a) homopolymerization of 20 and subsequent metalation with CuCl2; (b) copolymerization of 20 with EGDMA and subsequent metalation with CuCl2; or (c) molecular imprinting with the organometallic Mo-complex Mo(η3-C4H7)(CO)2(20) (28) and EGDMA and subsequent replacement of Mo(II) by Cu(II). All three polymeric Cu complexes were found to very efficiently promote the hydrolysis of activated phosphoesters with the relative activity being dependent on the nature of the polymer and the substrate. The complex Fe2O(CH3COO)(10)23 (42) was synthesized and structurally investigated by single crystal X-ray crystallography. After copolymerization with EGDMA and subsequent replacement of Fe(III) by Cu(II), Zn(II) or Mn(III), hydrolysis and oxidation reactions were investigated. A higher activity compared to the corresponding monometallic immobilized complexes was observed. This increase in activity is most likely caused by cooperative effects.

EPFL2006

Biomimetic catalysis with immobilized transition metal complexes

Alexander Schiller

EPFL2006

Etude des propriétés morphologiques et catalytiques d'agrégats d'or triés en taille et déposés sur le TiO2(110)

Raphaël Vallotton

EPFL2009